Luminescent and photoconductive liquid crystalline lamellar and helical network phases of achiral polycatenars

pi-Conjugated molecules are of special interest for designing new materials with potential optoelectronic applications. Herein, we report the synthesis of new functional non-symmetric substituted triple-chain pi-conjugated 5,5 '-diphenyl-2,2 '-bithiophenes. They have a long rod-like aromatic core unit with a 3,5-diheptyloxy substituted-pattern at one end and a single thioalkyl or alkoxy chain with variable length at the other end. Their molecular self-assembly was studied by polarized optical microscopy, differential scanning calorimetry, X-ray diffraction, luminescence and photoconductivity techniques. The shortest homologue forms an isotropic liquid phase with a broken mirror symmetry (Iso1[*]) beside smectic and 3D phases. Longer derivatives have no Iso1[*] or smectic components, and only exhibit 3D phases. The 3D phases are achiral double-gyroid bicontinuous cubic liquid-crystalline (LC) phases with a double helical network structure and Ia3d symmetry. Both the smectic and cubic phases exhibit photoconductivity, photoluminescence (PL), phosphorescence and electroluminescence at temperatures up to 150 degrees C. The smectic liquid crystals show weaker PL, but stronger photoconductivity; they could be polarized by an applied electric field, which turned a photoresistive device into both a photodiode and a light-emitting diode. Hence this work provides new functional bithiophene-based materials, which could be applied in optoelectronic devices. A prototype of such a device, namely a wavelength-measuring photodetector, is demonstrated.